The word courtly has nothing to do with a basketball court or court of law. Rather, courtly things relate to royal courts: they're sophisticated and grand.
Think about the pomp that surrounds a royal wedding in England, and you'll get a sense of the lavish, expensive, regal nature of courtly things. Nothing courtly is common, cheap, or average. Since kings like the finer things in life, courtly things are similarly extravagant and impressive.